Fireblight can infect plants with bacteria

The call comes into my office about a pear tree on which the leaves at the ends of the limbs are turning black. For the past two to three years, this has been a very common call.

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What is this problem that is attacking my trees? It is fireblight, a destructive, highly infectious and widespread disease caused by the bacterium Erwinia amylovora. Fireblight attacks blossoms, leaves, shoots, branches, fruits and roots. If fireblight is so prevalent, what can we do to control the disease?

First, we need to know what plants fireblight affects. Only plants that are in the family Rosaceae are affected. These include trees and shrubs such as flowering quince, cotoneaster, hawthorn, loquat, apple and crabapple, photinia, flowering plums and cherries, pyracantha, pear, rose and spirea. The plants that I see fireblight on the most are apple, crabapple and pear. Last summer, I saw fireblight on loquat for the first time.

How do you know if you have fireblight in your plants? Earlier I mentioned leaves at the end of limbs turning black or deep rust colored. When you look at a tree infected by fireblight from a distance, you can see the dead tips of the limbs from a distance. We call this flagging. Also, the branches at the end of the limb may be bent in the shape of a shepherd's crook. The bark at the base of the blighted leaves becomes water-soaked. These areas then turn dark, appear sunken and dry. Cracks can develop at the edge of the sunken area.

So, how does fireblight infect our plants? Fireblight enters the plant through natural openings such as flowers, or through cracks in the bark or other wounds. This year, we could see more fireblight due to the freeze we had at Easter. This cold weather caused the bark to split on many of our plants. Fireblight attacks new growth on the plant and then spreads to older growth. Fireblight can be spread from infected plants to healthy plants by rain, wind, insects and humans. The bacterium spends the winter in sunken cankers on infected branches. When the weather warms, the bacterium oozes out of the cankers. Bees and other insects are attracted to the bacterium, and they spread the disease to other plants. The bacterium starts to ooze about the same time that the plants start to flower.

If we have plants that are susceptible to fireblight, what can we do to prevent it or stop it once the plants are infected? The best option for control is to plant varieties that are resistant to fireblight. One of the questions that I get is why doesn't my neighbor's Bradford pear tree have fireblight? Well, the original Bradford pears are resistant to fireblight. However, Bradford pears have been replaced by other varieties of pears because of the problem with Bradford pears splitting or breaking apart. Most of the newer varieties of pears are not resistant to fireblight, but they are not subject to breaking apart. The second option for preventive control is to spray the trees during bloom with the antibiotic Agrimycin. Agrimycin can be hard to find. If you can find it, you need to spray the trees during the bloom stage. You will need to spray starting at bloom and repeat every three-four days while the tree is blooming. You can use a copper fungicide such as Kocide during the bloom. You need to spray with Kocide every seven days during the bloom. With either of these products, you need to repeat the application if it rains. Also, you need to make sure that you follow the directions on the label.

If your trees are infected with fireblight, you need to remove the infected limbs. The proper way to prune out the infect parts is to follow the infection down the limb until you come to the first green leaf on that limb. Once you find the first green leaf, you need to cut the limb off eight to 10 inches past the first green leaf. After you make the pruning cut, you need to disinfect your pruners after every cut. The best way to disinfect is with 70 percent rubbing alcohol or a 10 percent chlorine solution. You can make a 10 percent chlorine solution by mixing one part bleach with nine parts water.

If you don't disinfect your pruners, you will spread the disease each time you make a new cut. Afterwards, you need to clean and oil your pruners; the chlorine solution can damage them.

If your plants are infected by fireblight, you need to start pruning as soon as you see the disease. You will need to watch the plant closely and remove infected branches as they appear. Then next spring, you need to spray the trees that were infected.

Reach Columbia County Extension Agent Charles Phillips at (706) 868-3413 or charlesp@uga.edu, or the Extension at www.ugaextension.com/columbia.